As the use of navigation systems increases in both the public and military sectors, there is an incentive to improve the robustness and decrease the size of their individual components. One such component is the crystal oscillator, which supplies a stable clock frequency derived from the mechanical resonance of a piezoelectric crystal. Crystal oscillators can also be found in products such as test equipment, watches, and electronic circuits. Variants of the crystal oscillator engineered to reduce the impact of environmental factors such as temperature and humidity include the temperature-controlled (or -compensated) crystal oscillator (TCXO), the microcomputer-compensated crystal oscillator (MCXO) and the oven-controlled crystal oscillator (OCXO).
A TCXO typically includes a control chip electrically connected to the piezoelectric crystal oscillator. Traditionally, the control chip and the crystal are packaged in separate carriers which are then bonded together. The crystal is attached to its carrier with epoxy, and electrical connections are made between the two carriers. For example, the two carriers may be positioned one atop the other and soldered together. In some constructions, one end of the crystal is mounted inside its carrier using two small bumps of conductive epoxy. The two bumps provide both the support and electrical contacts for the crystal.
This scheme exposes the crystal to local stresses at the attachment point that can deleteriously affect its performance and reliability. For example, considerable stress may occur when the package is subjected to an inertial load or a harsh environment. If the elastic limits of the structure (or portions thereof) are exceeded, a permanent change in the TCXO frequency can occur.
Furthermore, the use of multiple carriers for a single oscillator package constrains its minimum size and can affect the performance of the oscillator as a function of temperature. For example, in the stacked packaging scheme discussed above, the thermal path between the crystal and its control chip is large, since it traverses both carriers. As a result, potentially harmful temperature fluctuations are more likely.
Finally, discrete packaging of the crystal and control components results in a large overall volume that can limit deployment.